Various proposals have hitherto been made on a microcapsule composed of a core material and an outer shell covering the core material. In applying microcapsules to electrophotographic toners, it is necessary to impart charge controlling properties to the capsule shell so as to visualize an electrostatic latent image. For this purpose, it has been proposed to apply a polymer containing a charge control agent to toner particles by spray drying or coating under heating or under pressure as disclosed in JP-A-57-202547, JP-A-63-27853, and JP-A-63-27854 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, the spray drying process involves a disadvantage that a plurality of toner particles are covered with a spray coating layer to produce large toner particles, which requires sieving to remove such large particles. After sieving, the yield of the toner of desired size is low. In addition, a large quantity of an organic solvent should be used, giving rise to a safety problem. The process of coating a polymer through heat fusion induces agglomeration of toner particles, also producing large toner particles. The process of coating a polymer under pressure is applicable to heat-fixable toner particles having sufficient hardness. However, where this process is applied to pressure-fixable capsule toners having a liquid core, the capsules would be destroyed.
JP-A-60-173552 describes a process for producing a toner, in which a coat comprising a binder resin containing a colorant, a magnetic powder or a conductive substance is formed on toner core particles by means of a jet mill. JP-A-63-49766 describes a process comprising mixing toner particles and a thermoplastic resin under strain to thereby coat the toner particles with the resin. Similarly to the above-mentioned cases, these processes are applicable to hard toner particles, for example, heat-fixable toners. However, when applied to capsule toners having a liquid core for pressure fixing, they result in destruction of the capsules.
JP-A-57-45558 proposes a process for producing a toner, in which toner core particles formed by polymerization are dispersed in an aqueous latex solution and a water-soluble inorganic salt is added to the dispersion to cause fine particles to be sedimented on the surface of the toner core particles thereby forming a coating layer. However, since the coating layer is formed of a deposit of fine particles on the core material, it has poor adhesion to the core material and poor covering power. Moreover, charging properties of the resulting toner are influenced by the surface active agent or inorganic salt remaining on the toner particles and are therefore greatly dependent on the environmental conditions. In particular, the chargeability is reduced under a high temperature and high humidity condition. JP-A-62-227161 and JP-A-62-227162 disclose a process for producing a capsule toner, in which a charge controlling polymer is formed by graft polymerization on the shells of microcapsules separately prepared by interfacial polymerization. However, since microcapsules prepared by utilizing interfacial polymerization have low properties of initiating graft polymerization, the above process requires a two-stage treatment; that is, a monomer having a plurality of vinyl groups per molecule is bonded to the capsule shell in the presence of a catalyst, e.g., a cerium salt, and then a monomer having charge controlling ability is graft-polymerized starting at the vinyl groups thus introduced.